1. In rabbit and cat retinas, synaptic interactions involving GABAergic and cholinergic amacrine neurons mediate certain specific types of ganglion cell receptive findings. In both species, GABA synaptic circuitry generates the antagonistic center-surrounding receptive fields of Y-type ganglion cells. In the rabbit, GABA amacrine cells, acting in concert with cholinergic amacrine cells, also generate direction- and orientation selective fields. Two questions concerning retinal organization may then be asked. First, what are the morphological characteristics of these GABAergic and cholinergic amacrine neurons? Second, what is the detailed synaptic circuitry through which these neurons exert their effects? These questions will be answered through the specific localization of GABAergic and cholinergic neurons in the two species. Specific antisera against glutamate decarboxylase and choline acetyltransferase will be used for the immunocytochemical localization of GABAergic and cholinergic amacrine neurons, and a morphometric analysis of the synaptic connectivity of these neurons will be carried out in both species. The long-range goal of these studies is an understanding of the detailed synaptic mechanisms underlying specific information-processing circuits in the mammalian visual system. 2. The hypothalamic peptide somatostatin serve as a neuroeffector in the central nervous system, and is present in some neurons in the retinas of many different species. The immunocytochemical localization of somatostatin will be used to answer two basic questions regarding the function of the peptide in this tissue. First, what are the detailed morphological characteristics of neurons that contain somatostatin? Second, what is the synaptic connectivity through which these cells act? These basic questions represent the first step in the analysis of the role of the neuropeptides in nervous system function.